The present invention relates generally to a non-combustible "paperless" gypsum board that is reinforced with organic fibers and to a process for making such a gypsum/fiber board. More particularly, the present invention relates to a gypsum/fiber board that can qualify as a non-combustible building material as specified by various building codes (e.g. BOCA) when tested in accordance the ASTM E136 test procedure.
Conventional gypsum wallboard or panel is typically manufactured from a plaster slurry wherein a wet slurry of calcium sulfate hemihydrate, generally referred to as calcined gypsum, is placed between two layers of paper and the slurry is allowed a certain amount of time to set. The set gypsum is a hard and rigid product obtained when the calcined gypsum reacts with water to form calcium sulfate dihydrate. Calcined gypsum is either calcium sulfate hemihydrate (CaSO.sub.4.multidot.1/2H.sub.2 O) or calcium sulfate anhydrite (CaSO.sub.4). When calcium sulfate dihydrate is heated sufficiently, in a process called calcining, the water of hydration is driven off and there can be formed either calcium sulfate hemihydrate or calcium sulfate anhydrite, depending on the temperature and duration of exposure. When water is added to the calcined gypsum to cause the gypsum to set, in essence, the calcined gypsum reacts with water, and the calcined gypsum is rehydrated.
In typical gypsum wallboard, the two layers of paper contain the slurry and provide the strength required in installation and use. The wallboard is cut into discrete lengths to accommodate subsequent handling and then dried in heated dryers until the board is completely dry. The bending strength of the wallboard depends largely on the tensile strength of the paper. The gypsum serves as the core and accounts for fire resistance and moisture absorbing and moisture releasing activities. The paper determines the nature of the joint filler system and the surface treatment that may be used on the board.
Although paper-covered wallboard has many uses and has been a popular building material for many years, the prior art has recognized that for certain applications it would be advantageous to provide gypsum panel that did not rely on paper surface sheets for strength and other properties. Several prior art fiber-reinforced gypsum panels are as follows:
U.S. Pat. No. 5,320,677 to Baig, which is incorporated by reference herein in its entirety, describes a composite product and a process for producing the product in which a dilute slurry of gypsum particles and cellulosic fibers are heated under pressure to convert the gypsum to calcium sulfate alpha hemihydrate. The cellulosic fibers have pores or voids on the surface and the alpha hemihydrate crystals form within, on and around the voids and pores of the cellulosic fibers. The heated slurry is then dewatered to form a matt, preferably using equipment similar to paper making equipment, and before the slurry cools enough to rehydrate the hemihydrate to gypsum, the matt is pressed into a board of the desired configuration. The pressed matt is cooled and the hemihydrate rehydrates to gypsum to form a dimensionally stable, strong and useful building board. The board is thereafter trimmed and dried. The process described in U.S. Pat. No. 5,320,677 is distinguishable from the earlier processes in that the calcination of the gypsum takes place in the presence of the cellulosic fibers, while the gypsum is in the form of a dilute slurry, so that the slurry wets out the cellulosic fibers, carrying dissolved gypsum into the voids of the fibers, and the calcining forms acicular calcium sulfate alpha-hemihydrate crystals in situ in and about the voids.
U.S. Pat. No. 5,342,566 to Schafer et al, which is incorporated by reference herein in its entirety, refers to a method of producing fiber gypsum board comprising the steps of mixing in a preliminary mixing step predetermined amounts of fibers and water respectively, to form a mixture of wetted, loose fibers; mixing in a mixing step the wetted fibers with a predetermined amount of dry calcined gypsum; premixing an accelerator with one of the components of dry calcined gypsum, fiber and water; promptly laying the mixed composition into a matt; immediately degassing the matt in a first compression step, adding a predetermined amount of water onto the resultant matt; and immediately compressing the matt to form a board composed of bonded fibers and gypsum. This process may be used to produce either a homogeneous board or a multilayered composite board. For example, the board can have outer layers of the same composition and a lighter core containing a lightweight filler, such as perlite. The board is preferably reinforced by fiber, such as paper fiber, although less fiber is included in the core layer. Several heterogeneous layers of board forming materials are placed on each other before the board is fully formed, pressed, and dried and wherein each of the layers is identical in composition.
U.S. Pat. No. 5,135,805 to Sellers et al, describes a water resistant gypsum product that may be a "faceless" product, i.e. it may not include a facing sheet of paper, fiberglass mat or similar material. The gypsum products described by U.S. Pat. No. 5,135,805 typically contain reinforcing fibers, for example, cellulosic fibers, such as wood or paper fibers, glass fibers or other mineral fibers and polypropylene or other synthetic resinous fibers. The reinforcing fibers can be about 10 to about 20 wt. % of the dry composition from which the set gypsum product is made. The density of such a product is typicality within the range of about 50 to about 80 pounds per cubic foot.
Carbo et al Provisional Application Ser. No. 60/073,503, describes a tri-layer, paperless gypsum/fiber board and a process for making such a gypsum/fiber board, all of which is incorporated by reference herein in its entirety.
Conventional wall board is classified as a non-combustible building material as specified by various building codes (e.g. BOCA) when tested in accordance the ASTM E136 test procedure. The building code allows the removal of 1/8 inch from both the top and bottom surface layers of the board in preparing the sample for the ASTM E136 test procedure. Conventional gypsum/fiber boards that are reinforced with organic fibers can not pass the E136 test because the presence of more than 3 or 4% of organic fibers in the core of the board render the board combustible. In the prior art, this problem was addressed through the use of non-combustible fibers, such as asbestos fibers, or by the inclusion of fire retarding agents such as aluminum trihydrate that reduced the amount of heat released during the test. Neither of these approaches produced satisfactory solutions.
Accordingly, it is yet a further object of the present invention to provide a gypsum/fiber board that is classified as a non-combustible building material as specified by various building codes (e.g. BOCA) when tested in accordance the ASTM E136 test procedure.
It is another object of the present invention to provide a building panel that has improved abuse resistance, better strength (MOR) in both directions and better nail pull strength as compared to conventional wall board.